Membrane processes have been widely applied in separation science, and can be applied to a range of separations of species of varying molecular weights in liquid and gas phases (see for example “Membrane Technology and Applications” 2nd Edition, R. W. Baker, John Wiley and Sons Ltd, ISBN 0-470-85445-6).
With particular reference to nanofiltration, such applications have gained attention based on the relatively low operating pressures, high fluxes and low operation and maintenance costs associated therewith. Nanofiltration is a membrane process utilising membranes with molecular weight cut-off in the range of 200-2,000 Daltons. Molecular weight cut-off of a membrane is generally defined as the molecular weight of a molecule that would exhibit a rejection of 90% when subjected to nanofiltration by the membrane.
Membranes for nanofiltration, pervaporation and gas separation are generally fabricated by making composite membranes. Thin film composite membranes may be fabricated via interfacial polymerization (herein also referred to as IP) or by coating [Lu, X.; Bian, X.; Shi, L., “Preparation and characterization of NF composite membrane.” J. Membr. Sci., 210, 3-11, 2002].
In glassy polymers, gas permeability depends strongly on the amount and distribution of free volume in the polymer (i.e. the space not occupied by polymer molecules) and on chain mobility. In liquid applications when using defect-free thin film composite membranes, high free volume leads to high permeability. Polymers with the highest permeabilities have rigid, twisted macromolecular backbones that give rise to microvoids. When the free volume is very high, these microvoids are interconnected resulting in intrinsic microporosity. Here, microporous materials are solids having interconnected pores of less than 2 nm in size [Handbook of Porous Solids, Schuth F, Sing K, Weitkamp J. Eds. Wiley-VCH; Berlin 2002, Vols 1-5]. This size of porosity is also commonly referred to as nanoporosity, and materials with this microporosity are referred to as being nanoporous.
To achieve very high permeabilities, high free volume and microporosity are sought after. Polymers presenting these properties are so-called high free volume polymers. These highly permeable polymers have been applied mostly to gas separations. Some examples include certain substituted polyacetylenes (e.g. PTMSP), some perfluoropolymers (e.g. Teflon AF), certain poly(norbornene)s, polymers of intrinsic microporosity, and some polyimides. Their microporosity has been demonstrated by molecular modelling and positron lifetime spectroscopy (PALS). Highly permeable polyacetylenes have bulky side groups that inhibit conformational change and force the backbone into a twisted shape. These rigid polymer macromolecules cannot pack properly in the solid state, resulting in high free volume. The free volume distribution comprises disconnected elements as in glassy polymers and continuous microvoids. In Teflon perfluoropolymers their high free volume is due to a high barrier to rotation between neighbouring dioxolane rings, coupled with weak interchain interactions, which are well known for fluoropolymers, leading to low packing density and hence high permeability. In the case of poly(norborene)s and PTMSP, the presence of bulky trimethylsilyl groups on the ring greatly restricts the freedom of the polymer to undergo conformational change. In polymers of intrinsic microporosity (PIMs), molecular linkers containing points of contortion are held in non-coplanar orientation by rigid molecules, which do not allow the resulting polymers to pack closely and ensure high microporosity. The PIMs concept has been reported for polymides [P M Budd and N B McKewon, “Highly permeable polymers for gas separation membranes, Polymer Chemistry, 1, 63-68, 2010].
There are two different types of PIMs, i) non-network (linear) polymers which may be soluble in organic solvents, and ii) network polymers which are generally insoluble, depending on the monomer choice. PIMs possess internal molecular free volume (IMFV), which is a measure of concavity and is defined by Swager as the difference in volume of the concave unit as compared to the non-concave shape [T M Long and T M Swager, “Minimization of Free Volume: Alignment of Triptycenes in Liquid Crystals and Stretched Polymers”, Adv. Mater, 13, 8, 601-604, 2001]. While the intrinsic microporosity in linear PIMs is claimed to derive from the impenetrable concavities given by their contorted structures, in network PIMs, microporosity is also claimed to derive from the concavities associated with macrocycles. In non-network PIMs, rotation of single bonds has to be avoided, whereas the branching and crosslinking in network PIMs is thought to avoid structural rearrangement that may result in the loss of microporosity (McKeown, 2010), so that single bonds can be present without loss of microporosity. In general, it has been observed that network PIMs possess greater microporosity than non-network PIMs due to their macrocyclization [N B McKewon, P M Budd, “Explotation of Intrinsic Microporosity in Polymer-Based materials”, Macromolecules, 43, 5163-5176, 2010]. However, since prior art network PIMs are not soluble, they can only be incorporated into a membrane if mixed as fillers with microporous soluble materials, which include soluble PIMs or other soluble polymers. There is a strict requirement in non-network PIMs that there are no single bonds in the polymer backbone, to prevent rotational freedom and so provide intrinsic microporosity. Highly rigid and contorted molecular structures are required, providing awkward macromolecular shapes that cannot pack efficiently in space. Molecules with awkward shapes are those that pose packing problems due to their concavities. However, in order to have microporosity in non-network PIMs, concave shape molecules are not sufficient as the voids must be sufficiently interconnected for transport to occur with minimal energy (i.e. intrinsic microporosity) [N B McKewon, P M Budd, “Explotation of Intrinsic Microporosity in Polymer-Based materials”, Macromolecules, 43, 5163-5176, 2010]. Non-network PIMs may be soluble, and so suitable for casting a membrane by phase inversion, or for use coating a support membrane to make a thin film composite. However, their solubility in a range of solvents restricts their applications in organic solvent nanofiltration [Ulbricht M, Advanced functional polymer membranes. Single Chain Polymers, 47, 2217-2262, 2006].
U.S. Pat. No. 7,690,514 B2 describes materials of intrinsic microporosity comprising organic macromolecules comprised of a first generally planar species connected by linkers having a point of contortion such that two adjacent first planar species connected by a linker are held in non-coplanar orientation. Preferred points of contortion are spiro groups, bridged ring moieties and sterically congested bonds around which there is restricted rotation. These non-network PIMs may be soluble in common organic solvents, allowing them to be cast into membranes, or coated onto other support membranes to make a thin film composite.
PIM-1 (soluble PIM) membranes exhibit gas permeabilities which are exceeded only by very high free volume polymers such as Teflon AF2400 and PTMSP, presenting selectivities above Robenson's 1991 upper bound for gas pairs such as CO2/CH4 and O2/N2. Studies have shown that permeability is enhanced by methanol treatment, helping flush out residual casting solvent and allowing relaxation of the chains [P M Budd and N B McKewon, D Fritsch, “Polymers of Intrinsic Microporosity (PIMs): High free volume polymers for membrane applications”, Macromol Symp, 245-246, 403-405, 2006].
A range of polyimides with characteristics similar to a polymer of intrinsic microporosity (PIM) were prepared by Ghanem et al. and membrane gas permeation experiments showed these PIM-Polyimides to be among the most permeable of all polyimides and to have selectivities close to the upper bound for several important gas pairs [B G Ghanem, N B McKeown, P M Budd, N M Al-Harbi, D Fritsch, K Heinrich, L Starannikova, A Tokarev and Y Yampolskii, “Synthesis, characterization, and gas permeation properties of a novel group of polymers with intrinsic micro porosity: PIM-polyimides”, Macromolecules, 42, 7781-7888, 2009].
U.S. Pat. No. 7,410,525 B1, describes polymer/polymer mixed matrix membranes incorporating soluble polymers of intrinsic microporosity as microporous fillers for use in gas separation applications.
International Patent Publication No. WO 2005/113121 (PCT/GB2005/002028) describes the formation of thin film composite membranes from PIMs by coating a solution of PIMs in organic solvent onto a support membrane, and then optionally crosslinking this PIM film to enhance its stability in organic solvents.
In order to improve the stability of soluble-PIMs membranes U.S. Pat. No. 7,758,751 B1, describes high performance UV-crosslinked membranes from polymers of intrinsic microporosity (PIMs) and their use in both gas separations, and liquid separations involving organic solvents such as olefin/paraffin, deep desulfurization of gasoline and diesel fuels, and ethanol/water separations.
Organic Solvent Nanofiltration (OSN) has many potential applications in manufacturing industries including solvent exchange, catalyst recovery and recycling, purifications, and concentrations. U.S. Pat. Nos. 5,174,899; 5,215,667; 5,288,818; 5,298,669 and 5,395,979 disclose the separation of organometallic compounds and/or metal carbonyls from their solutions in organic media. UK Patent No. GB 2,373,743 describes the application of OSN to solvent exchange; UK Patent No. GB 2,369,311 describes the application of OSN to recycle of phase transfer agents, and; European Patent Application EP1590361 describes the application of OSN to the separation of synthons during oligonucleotide synthesis.
Membranes for reverse osmosis and nanofiltration can be made by the interfacial polymerisation (IP) technique. In the IP technique, an aqueous solution of a first reactive monomer (often a polyamine) is first deposited within the porous structure of a support membrane, often a polysulfone ultrafiltration membrane. Then, the polysulfone support membrane loaded with the reactive monomer solution is immersed in a water-immiscible solvent solution containing a second reactive monomer, such as triacid chloride in hexane. The first and second reactive monomers react at the interface of the two immiscible solutions, until a thin film presents a diffusion barrier and the reaction is completed to form a highly cross-linked thin film layer that remains attached to the support membrane. The thin film layer can be from several tens of nanometers to several micrometers thick. The IP technique is well known to those skilled in the art [Petersen, R. J. “Composite reverse osmosis and nanofiltration membranes”. J. Membr. Sci, 83, 81-150, 1993]. The thin film is selective between molecules, and this selective layer can be optimized for solute rejection and solvent flux by controlling the reaction conditions, characteristics of the reactive monomers, solvents chosen and post-reaction treatments. The porous support membrane can be selectively chosen for porosity, strength and solvent stability. A particularly preferred class of thin film materials for nanofiltration are polyamides formed by interfacial polymerization. Examples of such polyamide thin films are found in U.S. Pat. Nos. 5,582,725, 4,876,009, 4,853,122, 4,259,183, 4,529,646, 4,277,344 and 4,039,440, the pertinent disclosures of which are incorporated herein by reference.
U.S. Pat. No. 4,277,344 describes an aromatic polyamide membrane produced by the interfacial polymerization of an aromatic polyamine with at least two primary amine substituents and an acyl halide having at least three acyl halide substituents. Wherein, the aqueous solution contains a monomeric aromatic polyamine reactant and the organic solution contains an amine-reactive polyfunctional acyl halide. The polyamide layer of TFC membranes is typically obtained via an interfacial polymerization between a piperazine or an amine substituted piperidine or cyclohexane, and a polyfunctional acyl halide as described in U.S. Pat. Nos. 4,769,148 and 4,859,384. A way of modifying reverse osmosis (herein also referred to as RO) TFC membranes for nanofiltration is described in U.S. Pat. Nos. 4,765,897; 4,812,270; and 4,824,574. Post-interfacial polymerization treatments have also been used to increase the pore size of TFC RO membranes. U.S. Pat. No. 5,246,587 describes an aromatic polyamide RO membrane that is made by first coating a porous support material with an aqueous solution containing a polyamine reactant and an amine salt. Examples of suitable polyamine reactants provided include aromatic primary diamines (such as, m-phenylenediamine or p-phenylenediamine or substituted derivatives thereof, wherein the substituent is an alkyl group, an alkoxy group, a hydroxy alkyl group, a hydroxy group or a halogen atom; aromatic secondary diamines (such as, N,N-diphenylethylene diamine), cycloaliphatic primary diamines (such as cyclohexane diamine), cycloaliphatic secondary diamines (such as, piperazine or trimethylene dipiperidine); and xylene diamines (such as m-xylene diamine).
In another method described in U.S. Pat. No. 6,245,234, a TFC polyamide membrane is made by first coating a porous polysulfone support with an aqueous solution containing: 1) a polyfunctional primary or secondary amine; 2) a polyfunctional tertiary amine; and; 3) a polar solvent. The excess aqueous solution is removed and the coated support is then dipped in an organic solvent solution of trimesoyl chloride (TMC) and a mixture of alkanes having from eight to twelve carbon atoms.
Many different types of polymers may be interfacially synthesized using interfacial polymerization. Polymers typically used in interfacial polymerization applications include, but are not limited to, polyamides, polyurea, polypyrrolidines, polyesters, poly(ester amides), polyurethanes, polysiloxanes, poly(amide imides), polyimides, poly(ether amides), polyethers, poly(urea amides) (PUA) [Petersen, R. J. “Composite reverse osmosis and nanofiltration membranes”. J. Membr. Sci, 83, 81-150, 1993]. For example, U.S. Pat. No. 5,290,452 describes the formation of a crosslinked polyesteramide TFC membrane produced via interfacial polymerization. The membrane is made by reacting a dianhydride (or its corresponding diacid-diester) with a polyester diol to produce an end-capped prepolymer. The resulting end-capped prepolymer is then reacted with excess thionyl chloride to convert all unreacted anhydride and all carboxylic-acid groups into acid chloride groups. The resulting acid-chloride derivative is dissolved in organic solvent and interfacially reacted with a diamine dissolved in an aqueous phase.
In order to improve the stability of TFC prepared by interfacial polymerisation, poly(esteramide) based TFC membranes have been developed showing improved oxidative (chlorine) resistance compared to polyamide membranes [M. M. Jayaraniand S. S. Kulkarni, “Thin-film composite poly(esteramide)-based membranes”, Desalination, 130, 17-30, 2000]. It has been reported that the rejection of polyesteramide TFC membranes can be tailored by varying the ester/amide ratio; more open TFC membranes were prepared using bisphenols with bulky substituents for diafiltration to separate organic molecules (MW>400 Da) from salts [Uday Razadan and S. S. Kulkarni, “Nanofiltration thin-film composite polyesteramide membranes based on bulky diols”, Desalination, 161, 25-32, 2004].
U.S. Pat. No. 5,593,588 describes a thin film composite reverse osmosis membrane having an active layer of aromatic polyester or copolymer of aromatic polyester and aromatic polyamide, which has improved chlorine-resistance and oxidation stability. The active layer is prepared by the interfacial polymerization of an aqueous solution of polyhydric phenol and a solution of aromatic acyl halide dissolved in organic solvent. Spiral-wound poly(ether/amide) thin film composite membranes designated PA-300, have been previously reported for water desalination applications. PA-300 was formed by an in situ interfacial polymerization of an aqueous solution of epichlorohydrin-ethylene diamine and an organic solution of isophthalyldichloride [R L Riley, R L Fox, C R Lyons, C E Milstead, M W Seroy, and M Tagami, “Spiral-wound poly(ether/amide) Thin-Film composite membrane systems”, Desalination, 19, 113-126, 1976].
The support membranes generally used for commercial TFC membranes made by interfacial polymerisation are often polysulfone or polyethersulfone ultrafiltration membranes. These supports have limited stability for organic solvents and, therefore, thin film composite membranes of the prior art which are fabricated with such supports cannot be effectively utilized for all organic solvent nanofiltration applications.
Although interfacially polymerized TFC membranes of the prior art have been specifically designed to separate aqueous feed streams down to a molecular level, they can be applied in certain organic solvents as well [Koseoglu, S. S., Lawhon, J. T. & Lusas, E. W. “Membrane processing of crude vegetable oils pilot plant scale removal of solvent from oil miscellas”, J. Am. Oil Chem. Soc. 67, 315-322, 1990, U.S. Pat. No. 5,274,047]. Their effectiveness depends on the specific molecular structure of the thin film layer and the stability of the support membrane. U.S. Pat. No. 5,173,191, suggests nylon, cellulose, polyester, Teflon and polypropylene as organic solvent resistant supports. U.S. Pat. No. 6,986,844 proposes the use of crosslinked polybenzimidazole for making suitable support membranes for TFC. TFC membranes comprising a thin film synthesized from piperazine/m-phenylenediamine and trimesoyl chloride on a PAN support membrane performed well in methanol, ethanol and acetone, less well in i-propanol and MEK, and gave no flux in hexane [Kim, I.-C., Jegal, J. & Lee, K.-H. “Effect of aqueous and organic solutions on the performance of polyamide thin-film-composite nanofiltration membranes.” Journal of Polymer Science Part B: Polymer Physics 40, 2151-2163, 2002].
US 2008/0197070 describes the formation of thin film composite membranes on polyolefin (e.g. polypropylene) supports prepared by interfacial polymerization. These membranes performed well in water, ethanol and methanol.
Non-reactive polydimethylsiloxane (PDMS) has been added during the interfacial polymerization reaction using polyacrylonitrile (PAN) as the support membrane [Kim, I. C. & Lee, K. H. “Preparation of interfacially synthesized and silicone-coated composite polyamide nanofiltration membranes with high performance.” Ind. Eng. Chem. Res. 41, 5523-5528, 2002, U.S. Pat. No. 6,887,380, U.S. Pat. Applic No. 0098274 2003]. The resulting silicone-blended PA membrane showed high hexane permeabilities.
TFC membranes have also been applied for filtration in apolar solvents. A method for the separation of lube oil from organic solvents (e.g. furfural, MEK/toluene, etc.) with a TFC membrane using poly(ethylene imine) and a diisocyanate on a solvent resistant nylon 6,6 support has been described in U.S. Pat. No. 5,173,911.
In interfacially polymerized composite membranes, both the surface chemistry and the morphology of the support membrane play a crucial role in determining the overall composite membrane performance. Membrane performance can be enhanced through modification of the membrane surface [D. S. Wavhal, E. R. Fisher, “Membrane surface modification by plasma-induced polymerization of acrylamide for improved surface properties and reduced protein fouling”, Langmuir 19, 79, 2003]. Thus, different procedures have been carried out to chemically modify the membrane surface and modify its properties. These procedures may increase the hydrophilicity, improve selectivity and flux, adjust transport properties, and enhance resistance to fouling and chlorine. Many methods have been reported for membrane surface modification such as grafting, coating [U.S. Pat. No. 5,234,598, 5,358,745, 6,837,381] and blending of hydrophilic/-phobic surface modifying macromolecules (SMMs) [B. J. Abu Tarboush, D. Rana, T. Matsuura, H. A. Arafat, R. M. Narbaitz, “Preparation of thin-film-composite polyamide membranes for desalination using novel hydrophilic surface modifying macromolecules”, J. Membr. Sci. 325, 166, 2008].
In order to improve the performance of TFC membranes, different constituents have been added to the amine and/or acyl halide solutions. For example, U.S. Pat. No. 4,950,404, describes a method for increasing flux of a TFC membrane by adding a polar aprotic solvent and an optional acid acceptor to the aqueous amine solution prior to the interfacial polymerization reaction. In a similar way, U.S. Pat. Nos. 5,989,426; 6,024,873; 5,843,351; 5,614,099; 5,733,602 and 5,576,057 describe the addition of selected alcohols, ketones, ethers, esters, halogenated hydrocarbons, nitrogen-containing compounds and sulfur-containing compounds to the aqueous amine solution and/or organic acid halide solution prior to the interfacial polymerization reaction.
It has been claimed that soaking freshly prepared TFC membranes in solutions containing various organic species, including glycerol, sodium lauryl sulfate, and the salt of triethylamine with camphorsulfonic acid can increase the water flux in RO applications by 30-70% [M. A. Kuehne, R. Q. Song, N. N. Li, R. J. Petersen, “Flux enhancement in TFC RO membranes”, Environ. Prog. 20 (1), 23, 2001]. As described in U.S. Pat. Nos. 5,234,598 and 5,358,745, TFC membrane physical properties (abrasion resistance), and flux stability can also be improved by applying an aqueous solution composed of poly(vinyl alcohol) (PVA) and a buffer solution as a post formation step during membrane preparation. Adding alcohols, ethers, sulfur-containing compounds, monohydric aromatic compounds and more specifically dimethyl sulfoxide (DMSO) in the aqueous phase can produce TFC membranes with an excellent performance [S.-Y. Kwak, S. G. Jung, S. H. Kim, “Structure-motion-performance relationship of flux-enhanced reverse osmosis (RO) membranes composed of aromatic polyamide thin films”, Environ. Sci. Technol. 35, 4334, 2001; U.S. Pat. No. 5,576,057; 5,614,099]. After addition of DMSO to the interfacial polymerization system, TFC membranes with water flux five times greater than the normal TFC water flux with a small loss in rejection were obtained [S. H. Kim, S.-Y. Kwak, T. Suzuki, “Positron annihilation spectroscopic evidence to demonstrate the flux-enhancement mechanism in morphology-controlled thin-film-composite (TFC) membrane”, Environ. Sci. Technol. 39, 1764, 2005].
However, in these prior art TFC membranes the use of a polysulfone support membrane limits the potential for additives to either aqueous amine solution or organic acid halide solution.
Several methods for improving TFC membrane performance post-formation are also known. For example, U.S. Pat. No. 5,876,602 describes treating the TFC membrane with an aqueous chlorinating agent to improve flux, lower salt passage, and/or increase membrane stability to bases. U.S. Pat. No. 5,755,965 discloses a process wherein the surface of the TFC membrane is treated with ammonia or selected amines, e.g., 1,6, hexane diamine, cyclohexylamine and butylamine. U.S. Pat. No. 4,765,879 describes the post treatment of a membrane with a strong mineral acid followed by treatment with a rejection enhancing agent.
A method of chemical treatment is claimed to be able to cause a simultaneous improvement of water flux and salt rejection of thin-film composite (TFC) membranes for reverse osmosis [Debabrata Mukherjee, Ashish Kulkarni, William N. Gill, “Chemical treatment for improved performance of reverse osmosis membranes”, Desalination 104, 239-249, 1996]. Hydrophilization by treating the membrane surface with water soluble solvent (acids, alcohols, and mixtures of acids, alcohols and water) is a known surface modification technique. This method increases the flux without changing the chemical structure [Kulkarni, D. Mukherjee, W. N. Gill, “Flux enhancement by hydrophilization of thin film composite reverse osmosis membranes”, J. Membr. Sci. 114, 39, 1996]. Using a mixture of acid and alcohol in water for the surface treatment can improve the surface properties, since acid and alcohol in water cause partial hydrolysis and skin modification, which produces a membrane with a higher flux and a higher rejection. It was suggested that the presence of hydrogen bonding on the membrane surface encourages the acid and water to react on these sites producing more charges [D. Mukherjee, A. Kulkarni, W. N. Gill, “Flux enhancement of reverse osmosis membranes by chemical surface modification”, J. Membr. Sci. 97, 231, 1994]. Kulkarni et al. hydrophilized a TFC—RO membrane by using ethanol, 2-propanol, hydrofluoric acid and hydrochloric acid. They found that there was an increase in hydrophilicity, which led to a remarkable increase in water flux with no loss in rejection.
A hydrophilic, charged TFC can be achieved by using radical grafting of two monomers, methacrylic acid and poly(ethylene glycol) methacrylate onto a commercial PA-TFC—RO membrane [S. Belfer, Y. Purinson, R. Fainshtein, Y. Radchenko, O. Kedem, “Surface modification of commercial composite polyamide reverse osmosis membranes”, J. Membr. Sci. 139, 175, 1998]. It was found that the use of amine containing ethylene glycol blocks enhanced the performance of the membrane, and highly improved membrane water permeability by increasing hydrophilicity [M. Sforga, S. P. Nunes, K.-V. Peinemann, “Composite nanofiltration membranes prepared by in-situ polycondensation of amines in a poly(ethylene oxide-b-amide) layer”, J. Membr. Sci. 135, 179, 1997]. Poly(ethylene glycol) (PEG) and its derivatives have been used for surface modification. TFC membrane resistance to fouling could be improved by grafting PEG chains onto the TFC—RO membranes [G. Kang, M. Liu, B. Lin, Y. Cao, Q. Yuan, “A novel method of surface modification on thin-film composite reverse osmosis membrane by grafting poly(ethylene glycol)”, Polymer 48, 1165, 2007, V. Freger, J. Gilron, S. Belfer, “TFC polyamide membranes modified by grafting of hydrophilic polymers: an FT-IR/AFM/TEM study”, J. Membr. Sci. 209, 283, 2002].
PEG has also been used to improve the TFC membrane formation [Shih-Hsiung Chen, Dong-Jang Chang, Rey-May Liou, Ching-Shan Hsu, Shiow-Shyung Lin, “Preparation and Separation Properties of Polyamide Nanofiltration Membrane”, J Appl Polym Sci, 83, 1112-1118, 2002]. Because of the poor hydrophilicity of the polysulfone support membrane, poly(ethylene glycol) (PEG) was added to the aqueous solution as a wetting agent. The effect of PEG concentration on the resulting membrane performance was also studied.
It has been reported that PEG is frequently used as an additive in the polymer solution to influence the membrane structure during phase inversion [Y. Liu, G. H. Koops, H. Strathmann, “Characterization of morphology controlled polyethersulfone hollow fiber membranes by the addition of polyethylene glycol to the dope and bore liquid solution”, J. Membr. Sci. 223, 187, 2003] The role of these additives is to create a spongy membrane structure by prevention of macrovoid formation and enhance pore formation during phase inversion. Other frequently used additives are: glycerol, alcohols, dialcohols, water, polyethylene oxide (PEO), LiCl and ZnCl2. US patent Nos. 2008/0312349 A and 2008/207822 A also describe the use of PEG in the polymeric dope solution during preparation of microporous support membranes.
It is generally known that heating, also known as curing, of thin film composite membranes can be required to facilitate the removal of organic solvent from nascent polyamide thin films, and to promote additional crosslinking by dehydration of unreacted amine and carboxyl groups. [Asim K. Ghosh, Byeong-Heon Jeong, Xiaofei Huang, Eric M. V. Hoek, Impacts of reaction and curing conditions on polyamide composite reverse osmosis membrane properties, Journal of Membrane Science 311 (2008) 34-45]. This heating or curing is usually undertaken after the interfacial polymerisation reaction, and can be in the range from 45° C. to 90° C. or higher.
The membrane products and membrane-related methods of the present invention advantageously address and/or overcome the obstacles, limitations and problems associated with current membrane technologies and effectively address membrane-related needs that are noted herein.